Further fixes for quoted-list GUC values in pg_dump and ruleutils.c.

Commits 742869946 et al turn out to be a couple bricks shy of a load.
We were dumping the stored values of GUC_LIST_QUOTE variables as they
appear in proconfig or setconfig catalog columns.  However, although that
quoting rule looks a lot like SQL-identifier double quotes, there are two
critical differences: empty strings ("") are legal, and depending on which
variable you're considering, values longer than NAMEDATALEN might be valid
too.  So the current technique fails altogether on empty-string list
entries (as reported by Steven Winfield in bug #15248) and it also risks
truncating file pathnames during dump/reload of GUC values that are lists
of pathnames.

To fix, split the stored value without any downcasing or truncation,
and then emit each element as a SQL string literal.

This is a tad annoying, because we now have three copies of the
comma-separated-string splitting logic in varlena.c as well as a fourth
one in dumputils.c.  (Not to mention the randomly-different-from-those
splitting logic in libpq...)  I looked at unifying these, but it would
be rather a mess unless we're willing to tweak the API definitions of
SplitIdentifierString, SplitDirectoriesString, or both.  That might be
worth doing in future; but it seems pretty unsafe for a back-patched
bug fix, so for now accept the duplication.

Back-patch to all supported branches, as the previous fix was.

Discussion: https://postgr.es/m/7585.1529435872@sss.pgh.pa.us

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